machinery and patients. In routine treatment planning, however, the deposition associated with secondary neutron doses is typically not taken into account. Complications in healthy tissues depend on both the irradiated volume and dose. Unfortunately, survivors of childhood cancer are at greater risk for the development of radiogenic secondary tumors than adult patients are. In fact, secondary malignancy development in childhood survivors of cancer ranges from 8% to 10% over 20 years. Second cancers pose serious challenges to increasing survival time and quality of life. Our goals are to quantify the secondary neutron exposures outside the treatment field for a variety of treatments at various beam energies in pediatric patients and to compile this information into a coherent, complete data set. The specific aims are to: 1. Measure the dependence of the neutron dose equivalent at various proton beam energies. Specifically, we will measure this at the treatment nozzle and just outside an irradiated water phantom using a neutron detector. We will study the produced neutron dose equivalent dependence on incident beam energy at 100, 120, 140, 160, 180, 200, 225, and 250 MeV. Additionally, we will place CR39 neutron dosimeters within the water phantom to measure the ambient neutron dose equivalent. 2. Model the neutron production from nuclear interactions between the proton beam, the relevant beam-line machinery, and the water phantom. We will compute the produced neutron dose equivalent per therapeutic absorbed dose using a Monte Carlo simulation code. Specifically, the calculations will focus on stray radiation originating from the treatment nozzle and within a spherical water phantom. 3. Evaluate the neutron dose equivalent outside of the treatment field. We will calculate the dependence of the neutron dose equivalent per therapeutic dose around the treatment nozzle at a variety of angles relative to the beam axis for different range-modulation widths. In doing so, we will estimate the fraction of produced secondary neutrons originating from the treatment nozzle in the neighborhood of the isocenter that can contribute to the therapeutic dose delivered to the patient during treatment. Additionally, we will estimate the in-patient neutron dose equivalent per therapy proton at the isocenter. Using these data, we will establish a rubric for the mean expected neutron dose deposited to healthy tissues in pediatric patients for various beam energies. [unreadable] [unreadable] [unreadable]